Metabolic adaptation and vertical zonation of Littorella uniflora (L.) Aschers. and Isoetes lacustris L.

The seasonal variation of photosynthesis and respiration was investigated in the two dominating plant species of contrasting vertical zonation in the oligotrophic Lake Kalgaard, Denmark, namely Littorella uniflora (L.) Aschers. (0-2 m) and Isoetes lacustris L. (2.0-4.5 m). The photosynthesis at a given temperature in low or high irradiance, and the chlorophyll content of Littorella and Isoetes were almost independent of season. In accordance with the zonation, the light-saturated net photosynthesis of Littorella was consistently higher compared with Isoetes and the difference increased with temperature (5-20°C). The photosynthesis in low irradiance, on the other hand, was often slightly, but not significantly, higher in Isoetes and, despite its higher chlorophyll content and lower specific leaf area, the slope of the initial part of the photosynthesis-light curve was rather similar for the two species. The dark respiration of the underground parts was lower than the leaf respiration and, in addition, the respiration was lower for Isoetes than for Littorella. The physiological responses are discussed in relation to the ecological conditions in oligotrophic lakes and to the distributional pattern of the two species. © 1978.